The use of a multi-stage continuous diluter as a component for sampling and diluting a process slurry for subsequent measurement of particle properties in the dilute state is described in our co-pending International Patent Application PCT/GB2004/000599 (Publication No. WO/2004/072603), the contents of which are incorporated herein by reference.
Such a multi-stage diluter offers a number of advantages over current dilution apparatus, for example it provides a continuously supplied dilute sample representative of the process that is delayed by only a few seconds between sampling and characterisation of the slurry. This allows ‘real-time’ process control.
Additionally, as sample is drawn through the diluter by a pressure drop in the diluent the diluter does not require pumps or other electrical apparatus to be close to the process, thereby increasing the safety of the diluter where a flammable solvent is used.
Such a diluter is advantageous for processes that are large scale and continuous, for example a pigment milling processes. In such large scale continuous processes the consumption of diluent is small compared to the volume of diluent consumed in the process itself. Also, diluted material can be returned to the process and re-absorbed with little or no impact on downstream product quality.
However, a continuous diluter also has drawbacks when applied to smaller scale or batch processes. For example each stage of a continuous diluter consumes typically 1-2 ltrs/min per stage and will extract approximately 150 mls of sample from the process per minute. With a diluter having from 2-6 stages in typical configurations the consumption of diluent can become a significant problem where it is expensive, toxic or environmentally damaging.
Such a continuous sampling regime can be unsuitable for processes where high volume and continuous consumption of diluent, and sample, are a disadvantage. For example, in small volume and batch processes involving high value material, for example a pharmaceutical, the diluted sample cannot be returned to the process without impacting the downstream processing. This means that either the sample must be discarded, or that the diluted sample requires special recovery means. In either case the cost of sampling the process is increased, and where special recovery means are used the complexity of the sampling arrangement is increased also.
Even in some high volume slurry processes continuous consumption of slurry is also a problem in that material is extracted from the process flow during times when no measurement is required. This means that slurry is being wasted thereby reducing the overall efficiency of the process.